High frequency coupling



May 1, 1962 c. FINK HIGH FREQUENCY COUPLING Filed March 16, 1960 INVENTOR ATTORNEYS United States Patent Oflice 3,032,726 HIGH FREQUENCY COUPLING Charles Fink, Silver Spring, Md., assignor, by mesne assignments, to Litton Industries of Maryland, Incorporated, Coliege Park, Md., a corporation of Maryland Filed Mar. 16, 1960, Ser. No. 15,316 16 Claims. (Cl. 333-=98) This invention relates generally to rotary couplings for microwave application, and more particularly, to an improved rotary coupling of the choke variety that is tunable for efficient operation over a wide range of frequencies.

Rotary couplings are employed for interconnecting a rotating antenna to a stationary microwave feed line and for many other applications where it is desired to convey a microwave signal from the line or source to a rotating member. Such couplings are required to provide a proper impedance match to both coupled members for all relative rotational positions therebetween, as well as to electrically couple the members in such manner as to eliminate any radio frequency leakage at the joint that may cause loss of electrical power and also provide spurious interference with other equipments in the vicinity. From a mechanical standpoint, it is also desired that such couplings be compact in size and weight and be capable of continuous rotational operation at speeds that may often exceed or rpm. without excessive wear to provide the necessary long life and dependable operation desired.

For performing this function prior art rotary couplings have been of two general types which may be classified as those having a contacting type of joint, such as sliding spring finger contacts or the like, and those having a noncontacting type of joint such as an electrical choke or microwave cavity at the joint to electrically couple the movable parts without physical contact therebetween. The former or contacting joint types have been generally found to be unsatisfactory in applications requiring high rotational speeds or where larger electrical power is to be transmitted through the coupling, since the engaging spring fingers or other contacting parts are subject to rapid wear at the high speeds as well as arcing or overheating when large electrical power is being transmitted through the sliding contacts. The latter or non-contacting joint types that employ a choke or microwave cavity have been found much more suitable for high speed and high power usage and under these conditions provide a much more dependable coupling having a longer life cycle of operation. However, since known couplings of the noncontacting types of joints depend upon the low input impedance of the choke or cavity, which impedance varies with frequency, such devices are relatively frequency sensitive and do not function properly where the frequency may be varied over a wide range. In fact, it has been found that even employing optimum engineering design, it is extremely diificut to obtain a choke coupling that can properly operate over a frequency bandwidth that may exceed 2 to 1.

To overcome this disadvantage and to provide a suitable non-contacting joint type of coupling for operation over a wider range or different frequencies, the present invention provides such a coupling having a variably adjustable choke or cavity portion which may be easily tuned, either manually or automatically as desired. According to one preferred embodiment, the tunable choke portions are arranged lengthwise of the coupled parts and are tuned to difierent frequencies by movement of the members lengthwise thereof all for the purpose of minimizing the overall size of the coupling and rendering the structure more compact. For the same purpose, the preferred choke portion is also arranged in a substantially U-shaped cross-sectional configuration and havinga mov- 3,032,726 Patented May 1, 1962 able divider wall therein whereby the effective electrical length of the cavity is made approximately twice as long as the outer lengthwise dimension thereof and the change in electrical length is correspondingly made twice as great as the physical displacement of the moving parts during tuning of the choke.

It is, accordingly, a principal object of the invention to provide a tunable rotary coupling for microwave applications enabling efiicient operation of the coupling over a wide band of frequencies.

A further object is to provide such a coupling that is compact in size and easily manufactured and assembled.

A still further object is to provide such a coupling having a low power loss at the joint and that is completely shielded to prevent any spurious power transmission at the joint.

Another object is to provide such a coupling that is rugged and dependable in construction and capable of conveying large electrical power to the rotatable members at high rotational speeds and with little wear or overheating.

Other objects, and many additional advantages, will be more readily understood by those skilled in the art after a detailed consideration of the specification taken with the accompanying drawing wherein:

FIG. 1 is a cross-sectional view, taken lengthwise, and illustrating a preferred embodiment of the invention, and

FIG. 2 is a section taken through lines 2-2 of FIG. 1.

Referring now to the drawings for a detailed consideration of a preferred embodiment of the invention, there is shown in FIGS. 1 and 2 a rotary coupling of the preferred type for interconnecting a fixed microwave transmission line of the coaxial variety to a rotational member (not shown) having input fittings that are also of the coaxial variety.

As shown, the coupling 10 is comprised of a rotational portion including an outer conductor part 11, at the left, and a coaxially disposed inner conductor part 15, both of which are adapted to be rotated together with respect to a second and stationary portion of the coupling at the right, and including an outer conductor 12 and a coaxial inner conductor 14. The left hand conductors 11 and 15 are adapted to be respectively connected to the conductor fittings on a rotational member (not shown), such as the lead-in coaxial line to the feed of a rotational scanning antenna, and the right hand coaxial conductors 12 and 14 are likewise adapted to be connected to the outer sheath and central conductor of a stationary coaxial transmission line.

For rotationally interconnecting the two portions of the coupling and electrically shielding the spaced joint 21 between the two outer conductors 11 and 12, there is provided an outer cylindrical shield member 18 which is integrally connected at its left hand end thereof to outer connector 11 for rotation therewith and at its right hand end is rotationally supported with respect to the stationary outer connector 12 by means of bearings 13. As shown, this outer shield 18 has a much greater diameter than the outer conductors 11 and 12 and extends lengthwise over appreciable portions of both outer connectors to fully shield the spaced joint 21 between the conductors, preventing any leakage of electrical energy to or from the joint area 21. The outer cylindrical shield 18 is also provided with sufficiently thick Walls to supply the necessary strength for interconnecting the fixed and rotary portions of the coupling, and the bearings 13 are likewise sufiiciently large and well seated in between the shield 18 and an outstanding shoulder ring 40 attached about the outer connector 12 to prevent any movement between the rotary and fixed portions of the coupling other than rotational movement about the lengthwise central axis of both coaxial po1tions. According to a preferred ems eaves E: bodiment, the, outstanding shoulder ring 40' is not necessarily a separate ring, as shown, but is preferably an outstanding flange portion integrally formed as part of the stationary portion of the coupling.

As generally indicated above, the outer conductor 11 of the rotary portion has a conductor part 19 projecting lengthwise and centrally into the shielded area and the end of conductor 19 does not abut but is rather spaced from the end of the stationary outer conductor 12 by an annular air gap 21 therebetween, which air gap 21 is maintained constantfor all rotational positions between the rotary portion and the stationary portion of the coupling.

The central conductor 15 of the rotary part makes continuous contact with the stationary central conductor 14 of the fixedpart by means of spring finger wiping contacts 16 or the like, engaging a tip or integral end portion 36 of the fixed central conductor 14. Even at relatively high rotational speeds, the wear and attrition at the wiping spring finger and tip portions is relatively small since such wear is a function of the linear velocity between these parts which is small due to the small diameters of these inner conductors.

As thus far described, therefore, there is provided a rotary coaxial line connector wherein the inner conductors are maintained in continuous electrical engagement by spring finger wiping contacts and the outer conductors are spaced apart by ,a constant annular air gap 21. According to the invention, the; annular air gap 21 or outlet between the two outer conductors Ill and 19 opens into a first annular passage 29 formed between the outside wall of the rotary outside conductor 19 and a cylinderzd of, greater diameter positioned coaxially thereof. The lefthandend of cylinder 24 is open and leads to a second annular passageway 30 formed between the outer wallof cylinder 2:4 and the inner wall of a second cylindrical member 23 of greater diameter than the first cylinderz l and ,coaxially positioned with respect thereto. Thus, the air gap 21 or outlet between the outer conductors :19 and 12 opens into two series-connected passages ZQ-and 3t? of generally U shaped cross-section and extending annularly about outer conductor 19. These two passages 29 and Share each substantially of equal length in a direction lengthwise of the coupling members and together serve as a choke or cavity leading from the outlet 21. If the total length of the sum of these two connected passages is made equal to one-half the Wave length of the electrical signal being transmitted through the coupling, it isevident that the air gap 21 is electrically short circuited, and the signal passes from outer conductor 12 of the Stationary portion to the outer conductor part 19 of the rotary portion without any appreciable mismatch as if these members were physically connected together. This is based on the fact that the input impedance to a short circuited half wave transmission line or cavity is zero, neglecting any losses in the line, and hence the electrical impedance of gap 21 is likewise substantially zero if the overall length of passages 29 and 3t} equals one-half the wave length of the frequency.

To provide the series connected passages 29 and 30, as described above, the second outer cylindrical member 23 is made of greater length than the first cylindrical member 24, as shown, to extend beyond the end of the first cylinder 24, and is provided with an inwardly disposed flanged or base portion at the left-hand end thereof that encircles the outer conductor part 19 but is spaced therefrom by a small annular air gap 38. The lengthwise distance from the rotary point outlet or air gap 21 to the small air gap 38 is made approximately one-quarter of'the wave length at the frequency being transmitted, whereby, insofar as the electrical signal in the cavity is concerned, it is not necessary that physical contact be made between the second cylinder 23 and the conductor 19 at the gap portion 38. This results from the fact that in a cavity or transmission line of one-half wavelength, the

t current flow is high at the beginning of the line and high at the end or at one-half wavelength spaced portion but substantially zero at the quarter wavelength portion. Consequently, if the gap 33 occurs at the quarter wavelength portion, it is not necessary to provide good electrical contact since no current is flowing at that portion.

As indicated above, however, the impedance at rotary joint air gap 21 existing between the outer conductors 19 and 12 is only substantially zero when the combined length of the two passages 29 and 30 equals one-half wavelength, and if the frequency of the signal passing through the coupling is varied, the impedance at air gap progressively increases with change of frequency, rendering this choke coupling frequency sensitive. It has been found that if the dimensions of the passages 29 and 3d are fixed in length, the maximum permissible change in frequency that can be tolerated for most applications is over bandwidths of less than 2 to 1 since, for greater frequency variations, the impedance mismatch is generally unacceptable.

To overcome this shortcoming and enable the rotary coupling to beemployed for applications requiring greater frequency deviation as well as operating more eificiently with smaller frequency deviations, means'are provided according to the invention for tuning the choke or varying the length of the overall cavity or choke sections. Returningto FIGS. 1 and 2, the choke sections or passages 29 and 30 are simultaneously variable by supporting the first and second cylindrical sections 24 and 23 for reciprocal displacement lengthwise along the coupling. More specifically, the first cylindrical member 24 is slidably accommodated within an elongated annular opening 42 formed lengthwise through the structure of fixed outer conductor 12, whereby this cylinder 24 may be reciprocally positionedcoaxially alongside the outer conductor 12 and extend beyond the end of conductor 12, past the air gap 21, for variable distances over conductor 19 to vary the lengthof the first passage 29. For reciprocally positioning the cylinder 24 in this manner, a plurality of push rods 32 are disposed in spaced relation about the right hand end of cylinder 24 and connected to this end portion ,of cylinder 24. These rods 32 project through suitable openings 43 in the fixed portion of the coupling, and their opposite or right hand ends are connected to an actuator ring or apertured disc 34 that is disposed about the outside of conductor 12, and is reciprocally slidable lengthwise along the fixed conductor 12. Actuator ring 34 may be manually slided along the outside of coaxial conductor 12 or maybe positioned by means of a motor actuator (not shown) that is connected to ring 34 at eyelet connectors 35.

The second or outer cylinder 23 is also supported for reciprocal movement lengthwise of the outer conductors 12 and 19 and in unison with the first cylinder 24 by means of push rods 33 also connecting at their opposite ends with the actuator disc 34. Consequently, both cylinders 23 and 24 are connected for sliding movement in unison alongside the outer conductors l2 and 19, whereby they simultaneously vary the lengths of the inner and outer passages or cavities 29 and 30 together and by equal distance increments. It is to be particularly noted that the inner and outer cylinders 24 and 23 are supported by the fixed or right hand portion of the coupling 1t) and consequently, do not rotate with the rotary lefthand portions thereof, including conductors 11 and 15 and outer shield 18. For this reason, these cylinders 24 and 23 may be easily supported for the reciprocal displacement desired; the inner cylinder 24 being retained by the push rods 32 and leaf finger wiping contacts 28 attached to the fixed portion of the coupling part, and the second cylinder 23 being slidably held by means of the push rods 33, suitable bearings 26 and the leaf fingers 27. The leaf finger contacts 27 and 28 also insure that good electrical contact is continuously made between the fixed portion of the coupling and the slidable cylinders 24-and 23.

Recapitulating briefly, the functioning of the cylinders 24 and 23 for enabling the choke or cavities to be varied in length as desired for different frequency operation, the outer or second cylinder 23 is made longer to extend beyond the first cylinder 24, and is provided with an inwardly disposed flange part 41 to provide an end wall interconnecting the two annular passages 29 and 30. Both cylinders are supported by the fixed portion of the housing and made reciprocally slidable together lengthwise of the conductors 12 and 19. As the cylinders 24 and 23 are moved to the left, further alongside of conductor 19, the length of the passages 29 and 30 are increased, and as both are moved to the right, the length of passages 29 and 30 is decreased. Since the passages 29 and 30 are series connected and in U-shaped cross-section, any unit displacement of the cylinders 24 and 23 to the right or left results in twice the unit change in the total length of the choke, since each passage is varied in length by one unit and the combined change in length is therefore double. This feature is particularly important for providing a wide range of tuning adjustment in a rotary coupling of minimum size and weight.

What is claimed is:

1. A variable frequency rotary joint coupling for a microwave frequency transmission line comprising a fixed portion for connection to the line and having a central conductor and an outer conductor coaxial therewith, a rotatable portion having a central conductor for electrical engagement with the central conductor of the fixed portion, and an outer conductor coaxial therewith and in longitudinal alignment with the outer conductor of the fixed portion but in non-abutting relationship therewith to provide an air gap therebetween forming an outlet for the microwave energy, and means providing an electrical short circuit between said non-abutting outer conductors over a wide range of frequencies without physical contact between the outer conductors, said means including an adjustable frequency choke section positioned to receive energy from said outlet, and means enabling the adjustment of said section to one-half the wavelength of the microwave frequency over a wide range of microwave frequencies.

2. In the rotary joint of claim 1, said adjustable choke section being disposed lengthwise of said outer conductors to reduce the transverse dimensions of said coupling.

3. In the rotary joint of claim 1, said choke section being disposed lengthwise of said outer conductors and coaxial therewith, and being adjustable in length lengthwise of said conductors.

4. In the rotary joint of claim 3, said choke section including a lengthwise divider to double the electrical length of the choke and physically shorten its outer dimensions.

5. In the coupling of claim 1, said choke section including a first elongated member supported by one of said coupling portions and coaxial with said outer conductors and spaced therefrom to form a chamber opening from said outlet, and adjustable means supporting said member for lengthwise movement relative to said outer conductors to vary the length of said chamber.

6. In the coupling of claim 5, said member being disposed completely around said outer conductors.

7. In the rotary joint of claim 6, a second member disposed between said member and said outer conductors to divide the chamber into two portions and effectively double its length.

8. In the device of claim 7, said member and second member being jointly slidable lengthwise of said outer conductors whereby for each incremental displacement of said conductors said chamber length is varied a distance equal to substantially twice of said incremental displacement.

9. In the device of claim 8, said outer conductors being cylindrical in shape and of the same diameter, and said member and second member being cylindrical in shape with said member being of greater diameter than said conductors and said second member being of greater diameter than said conductors but less than said member.

:10. In the device of claim 9, said member being longer than said second member and having an inwardly disposed flange extending past the end of said second member to substantially abut against the outer surface of one of said outer conductors but being spaced therefrom, thereby to form with said second member a substantially U-shaped chamber in cross-section disposed annularly about the conductors and opening from said outlet.

11. In the rotary joint of claim 1, said variable frequency choke section including two coaxially spaced cylindrical members of different length slidable in unison lengthwise of the outer conductors, with the longer of said members being provided with an inwardly disposed flange encircling one of said outer conductors in non-abutting relation, whereby said members provide a substantially U-shaped cavity in cross-section annularly about said outer conductors and opening from said outlet, and means enabling the positioning of said members in unison thereby to adjust the length of said cavity to one-half wavelength of the frequency over the transmission line.

12. In a rotary joint of claim 11, the distance from said outlet to the position where said flange encircles said outer conductor being substantially one-half the length of said cavity whereby when said choke section is adjusted to onehalf the wavelength of the frequency said distance equals substantially one-quarter wavelength.

13. A variable frequency rotary coupling for a microwave transmission line comprising a stationary hollow conductor and a rotatable hollow conductor in axial alignment therewith and supported for rotation with respect thereto, said rotatable conductor being axially spaced from said fixed conductor by a small gap therebetween, and tunable means providing an electrical short circuit across said gap over a wide band of frequencies, said means including an adjustable frequency choke section to receive energy from said gap, and means movably supporting the parts of said choke section with respect to said stationary portion.

14. In the rotary coupling of claim 13, said adjustable frequency choke section including a pair of coaxially disposed hollow members of different length slidably supported by said fixed portion and extending outside of and alongside said stationary and rotatable outer conductors to provide a pair of series connected passages leading from the gap and having a U-shaped cross section, and means enabling said members to be variably positioned in unison lengthwise said conductors thereby to vary the length of said passages.

15. A frequency tunable rotary joint for microwave transmission comprising: a fixed electrically conducting member and a rotary conducting member positioned in substantially coaxial alignment and spaced apart from one another to form an outlet, a third member spaced about said first and second members and forming a cavity from said outlet, and means for positioning said third member in a direction lengthwise along said first and second members to reversibly increase and decrease the length of the cavity.

16. A frequency tunable rotary joint for microwave transmission comprising: a pair of axially aligned conductive members spaced apart from one another to form an outlet and being relatively rotatable about a common axis, a concentrically disposed outer arcuate member being slidably supported with respect to one of said mem bers and extending lengthwise over said outlet with a portion thereof projecting coaxially alongside the other conductor, said outer member having a surface portion spaced from said outlet and from said other conductor and a flanged portion thereof that is electrically coupled to said other conductor and not in physical contact therewith to provide an electrical short-circuit thereto, a concentrically arranged intermediate arcuate member positioned between said conductive members and said outer spear/2e r arcuate member and being spaced from said members, said References Cited in the file of this patent intermediate arcuate member being slidably supported UNITED STATES PATENTS With respect to one of said conductive membersrand extending lengthwise over said outlet With a portion thereof P 1948 ro'ectin coaxiall' alon side the other conductor, said Salisbury 1948 P J g y g 5 2,853,681 Small Sept. 23, 1958 intermediate member and outer arcuate member providing a U-shaped cavity chamber leading from said outlet, OTHER REFERENCES and means for axially positioning said intermediate arcuate member and outer arcuate member in unison thereby to vary the length of said chamber in a ratio that is twice 10 as great as the axial displacement of said members.

Ragan: Microwave Transmission Circuits 7 Radiation Laboratory Series, vol. 9, MeGravwHill, page 410. 

